1. Field of the Invention
The present invention relates generally to aircraft skid landing gear having a various directional stiffnesses. More specifically, the present invention relates to a skid landing gear assembly for a helicopter in which the vertical stiffness is de-coupled from the longitudinal stiffness.
2. Description of Related Art
Helicopter skid landing gear devices are well known in the art. For instance, most modern helicopters feature a skid landing gear having a pair of circular-section cross members attached to a pair of longitudinal skids. These circular-section cross members are designed to optimize the attenuation of the vertical energy of hard landings and to maximize fatigue life. However, with these circular-section cross members, the vertical stiffness is inherently coupled to the longitudinal stiffnesses, resulting in relatively high longitudinal stiffnesses. It is well known in the art that some of the rotor blade's rotational energy is transformed to oscillatory energy of the fuselage and of the rotor blade in the in-plane mode. This phenomenon is known as ground resonance. Ground resonance is destabilizing and requires adequate system damping in the fuselage and skids and in the rotor blade's lead-lag dampers. When the system damping is less than or equal to zero, there is a potential for instability. A high longitudinal stiffness generally has an adverse affect on shuffle mode ground resonance frequency placement, creating a direct conflict between energy attenuation and fatigue life requirements and ground resonance frequency placement. In order to resolve this conflict, prior-art skid landing gear designers have resorted to heavy and costly add-on devices, such as rocker beams, dampers, and skid springs.
For example, U.S. Pat. No. 4,270,711 to Cresap et al. discloses a helicopter skid landing gear with cross tube pivot. Cresap et al. employs a structural beam, or rocker beam, that allows central pivoting of the aft cross tube, creating a three-point support for reducing roll frequency. Cresap et al. has only an indirect affect on the longitudinal shuffle frequency. Thus, Cresap et al. is limited in its ability to tune the skid gear for both attenuating vertical landing energy, and avoiding ground resonance frequency. The Cresap et al. device is costly, complicated, and heavy. In addition, it is difficult to design the centrally pivoted aft cross tube for adequate fatigue life.
Another example is U.S. Pat. No. 5,211,359 to Rene et al., which discloses a landing gear for aerodynes with cross pieces made of composite material. The Rene et al. device features a skid gear with laminated fiberglass cross members. Although it is generally believed that energy absorption cannot occur elastically, Rene et al. discloses an elastic energy absorption capacity that is superior to metallic devices. Although Rene et al. discloses the use of dampers to control ground resonance, there is no mention of how the dampers are tuned for ground resonance frequency placement. Rene et al. does not disclose the use of its laminated cross member section properties to obtain compliance to avoid ground resonance.
Another example of a helicopter skid landing gear is disclosed in U.S. Pat. No. 5,224,669 to Guimbal. The Guimbal device features laminated cross members made in the shape of an arch. Guimbal capitalizes upon the non-linear geometry of the arches and friction to obtain vertical energy absorption for light helicopters. Although Guimbal discloses the use of dampers to control the ground resonance, it makes no mention of how to tune the system properties for ground resonance stability.
Although these prior-art devices teach various methods of absorbing the vertical energy of landing, they do not adequately address the problem of helicopter ground resonance frequency placement. Despite these advances in the art, there continues to be a need for a helicopter skid landing gear that addresses not only vertical energy attenuation and fatigue life, but that also adequately addresses the problem of controlling the ground resonance frequency.